Particulate Organic Matter Composition and Distribution in the Twilight Zone

The efficiency of the biological pump in transferring CO₂ from the atmosphere to the deep ocean strongly depends on the processing of particulate organic matter (POM) in the water column, particularly in the “Twilight Zone” (100 – 1000 m). However, the processes which control the efficiency of carbon storage are not well known. We provide some insight by using elemental, bulk nitrogen and carbon stable isotopic and lipid biomarker compositions of fractionated POM collected from contrasting regions of the Southern Ocean, South Georgia and the Benguela Upwelling.

POM concentrations decrease rapidly with depth, more so at the Benguela Upwelling than at South Georgia. The OC/TN ratios of the small particles (<53 µm) decrease with depth from ~5 to ~3.5 in South Georgia and ~5.5 to ~4 in the Benguela Upwelling Region, implying that C is remineralised more rapidly than N at both locations. In South Georgia between 50 m and 250 m, POM δ¹³C values decrease from -20‰ to -22‰, indicating selective remineralisation of ¹³C-rich molecules e.g. carbohydrates. The δ¹⁵N values of surface POM (4.3‰), reflect the uptake by phytoplankton of enriched nitrate. The δ¹⁵N values of POM in the Benguela upwelling increase from 2‰ to 7‰ from the surface to 100 m, indicating rapid recycling of POM.

Total lipid concentrations decrease with depth in both regions, being dominated by eicosapentenoic acid (EPA) and docosahexenoic acid (DHA). These labile phytoplankton-derived compounds contribute 22 to 53% of the < 53 µm POM lipids. At South Georgia, compounds specific to zooplankton dominate large particles (> 53 µm) throughout the water column and contribute up to 36% of the total lipid in surface water samples. The greatest attenuation of labile lipid is in the upper 150 m of the water column implying rapid turnover of POM there. Concentrations do not decrease significantly below this and there is labile material available for respiration to 500 m.